Speed up nmod_addmul and fullword _nmod_vec_scalar_mul_nmod, _nmod_vec_scalar_addmul_nmod

[fredrik-johansson]

nmod_addmul is replaced with simpler and faster code which simplifies and speeds up the generic fallback in _nmod_vec_scalar_addmul_nmod. The old code for 64-bit moduli in _nmod_vec_scalar_addmul_nmod was particularly bad; the new code is more than 3x faster.

Also, nmod_redc_mul is used to speed up the 64-bit case in _nmod_vec_scalar_mul_nmod where Shoup can't be used. Asymptotically this is about 20% faster than the old code, with the only restriction is that n needs to be odd to benefit.

Timings on Zen 3:

           vec_mul               vec_scalar_addmul            speedup
bits len   old       new         old        new             mul     addmul
63   1     4.66e-09  4.69e-09    4.21e-09   4.23e-09        0.994   0.995
63   2     6.02e-09  6.02e-09    6.14e-09   6.92e-09        1.000   0.887
63   3     6.77e-09  6.79e-09    7.37e-09   7.89e-09        0.997   0.934
63   4     8.52e-09  8.52e-09    1e-08      9.85e-09        1.000   1.015
63   5     1.04e-08  1.03e-08    1.16e-08   1.12e-08        1.010   1.036
63   6     1.18e-08  1.17e-08    1.27e-08   1.26e-08        1.009   1.008
63   7     1.25e-08  1.26e-08    1.48e-08   1.42e-08        0.992   1.042
63   8     1.46e-08  1.44e-08    1.71e-08   1.59e-08        1.014   1.075
63  10     1.22e-08  1.21e-08    1.53e-08   1.59e-08        1.008   0.962
63  12     1.28e-08  1.28e-08    1.77e-08   1.76e-08        1.000   1.006
63  15     1.58e-08  1.58e-08    2.14e-08   2.14e-08        1.000   1.000
63  18     1.7e-08   1.69e-08    2.53e-08   2.52e-08        1.006   1.004
63  22     1.98e-08  1.99e-08    2.95e-08   2.98e-08        0.995   0.990
63  27     2.38e-08  2.34e-08    3.74e-08   3.73e-08        1.017   1.003
63  33     2.81e-08  2.77e-08    4.51e-08   4.5e-08         1.014   1.002
63  41     3.26e-08  3.25e-08    5.52e-08   5.59e-08        1.003   0.987
63  51     4.01e-08  4.01e-08    6.94e-08   6.88e-08        1.000   1.009
63  63     4.84e-08  4.83e-08    8.41e-08   8.41e-08        1.002   1.000
63  78     5.86e-08  5.86e-08    1.03e-07   1.02e-07        1.000   1.010
63  97     7.24e-08  7.24e-08    1.28e-07   1.28e-07        1.000   1.000
63 121     8.99e-08  9e-08       1.61e-07   1.6e-07         0.999   1.006

64   1     3.9e-09   3.78e-09    4.01e-09   4.14e-09        1.032   0.969
64   2     5.01e-09  5.29e-09    6.01e-09   5.44e-09        0.947   1.105
64   3     5.8e-09   6.43e-09    8.85e-09   6.57e-09        0.902   1.347
64   4     7.18e-09  7.17e-09    1.21e-08   7.8e-09         1.001   1.551
64   5     8.7e-09   8.76e-09    1.47e-08   9.2e-09         0.993   1.598
64   6     1.03e-08  1.02e-08    2.26e-08   1.05e-08        1.010   2.152
64   7     1.11e-08  1.17e-08    2e-08      1.18e-08        0.949   1.695
64   8     1.41e-08  1.39e-08    3.46e-08   1.32e-08        1.014   2.621
64  10     1.6e-08   1.6e-08     3.19e-08   1.6e-08         1.000   1.994
64  12     1.85e-08  1.88e-08    4.1e-08    1.87e-08        0.984   2.193
64  15     2.22e-08  2.19e-08    7.66e-08   2.31e-08        1.014   3.316
64  18     2.7e-08   2.44e-08    8.16e-08   2.72e-08        1.107   3.000
64  22     3.22e-08  2.9e-08     9.9e-08    3.3e-08         1.110   3.000
64  27     3.89e-08  3.64e-08    1.45e-07   4.07e-08        1.069   3.563
64  33     4.71e-08  4.34e-08    1.68e-07   4.83e-08        1.085   3.478
64  41     5.9e-08   5.25e-08    2.21e-07   5.94e-08        1.124   3.721
64  51     7.29e-08  6.26e-08    2.82e-07   7.3e-08         1.165   3.863
64  63     8.93e-08  7.72e-08    3.34e-07   9.48e-08        1.157   3.523
64  78     1.11e-07  9.12e-08    4.3e-07    1.16e-07        1.217   3.707
64  97     1.37e-07  1.14e-07    5.47e-07   1.43e-07        1.202   3.825
64 121     1.72e-07  1.41e-07    6.96e-07   1.77e-07        1.220   3.932